Unfortunately that’s just how it is with pixel art. We can’t rotate shapes perfectly as there just aren’t enough pixels to maintain the shape. Even when you rotate high resolution images in for example GIMP or Photoshop there is still quality loss, although it’s not as noticeable as it interpolates the pixels to smooth it out.
This, but it’s made worse when compounded. Rotating an already rotated sprite means the program has less accurate “information” about where the pixels “should” be, and this compounds with every iteration.
@Paddy, you can mitigate the effect by not performing your rotation over and over to the previous rotation, but instead start from the original sprite on each frame and rotate the appropriate amount for each frame.
I actually came here to say that the dev advised me to do exactly that over on the Steam forums! Great minds think alike
Using the initial image as the “base” for all rotation steps does indeed yield much better, much more workable results. It would be great if the program had a way of doing this itself, but for the time being the manual method is a very usable workaround that saves a lot of time/effort by reducing the amount of retouching needed.